Recombination of chromosome
The process, which produces recombinations of gene by interchanging of corresponding segments between nonsister chromatids of homologous chromosomes, is called recombination of chromosomes. It takes place in pachytene stage of prophase I of meiosis. Crossing between the linked genes results in genetic recombination.
According to Bateson and Punnet, in Lathyrus odoratus 12 per cent of the test cross progeny were recombinants. Recombination between two genes is expressed in percentage. It is called recombination frequency. Gene pairs that had very low percentage of recombination are known as tightly linked genes. The gene pairs with higher percentage are termed as loosely linked genes. For example, 12 per cent of the test cross progeny were recombinants. They showed a different linkage of alleles than their parents. The percentage recombination is determined by dividing the number of recombinant offspring by the total number of offspring. In the figure 3.4, the linkages of the parents were B with L and b with l. The recombinant offspring are B with l or b with L.
Chromosomes are the physical carriers of genes, which are made up of DNA and associated proteins. The term chromosome was introduced by Waldeyer in 1888. Chromosomes occur in all the living organisms. The bacterial chromosomes are circular. It has closed circular DNA. Linear chromosomes are found in eukaryotes. Bridges in 1916 was the first to prove that the genes are carried on the chromosome.
Linkage and crossing over
The tendency of genes or characters to be inherited together because of their location on the same chromosome is called linkage. Many hybridization experiments were conducted both on plants and animals based on Mendel's work. The results of certain dihybrid crosses did not confirm the law of independent assortment. It states that the inheritance of genes of each pair in a dihybrid during gamete formation is independent of the other.